The detection of the presence of, and frequently also the measurement of the concentration of, various gases, environmental pollutants, and toxic gases, is of increasing importance. While the presence and concentration of these can usually be accomplished by the use of conventional sampling and analytical techniques, many of the situations which they represent require very rapid accomplishment, and rugged and reliable devices for the purpose. It is no longer suitable occasionally to sample stack gas or ambient air and then in what was once a reasonable time to read out what the conditions were. Frequently these situations if not corrected can result in costly waste of fuel, pollution of the atmosphere which may give rise to penalties, or to the unwarned presence or emission of toxics.
As one example, operators of combustion devices such as boilers are well aware that continuous detection and measurement of gases produced in minor quantities such as carbon monoxide, and responsive control of the processes which produce them, can result in dramatically improved fuel efficiency. In such installations excess air was formerly widely used in combustion processes on the assumption that a lean mixture would assure more complete combustion of fuel. However, as combustion processes became better understood, it also became apparent that the use of excess air was wasteful, because among other things it required the flame to heat excess gas, enabled the formation of SO.sub.3 instead of merely SO.sub.2, encouraged the formation of NO, created sulfate emissions, and in some cases even increased smoke formation by shortening the flame length. Combustion operations using low excess air improve all of the above situations, but the control must be accurate, and be quickly responsive in order to insure complete combustion while avoiding uneconomical operations and the formation of excessive pollutants. The concentration of carbon monoxide produced by a combustion process turns out to be a good measure of the average combustion quality, i.e., nearness to stoichiometric conditions. For example, no CO means too much air, while high CO means not enough air.
With the realization that controls based on the concentration of some minor component of a gas stream can lead to an optimized combustion function, serious development of suitable instrumentation was undertaken, especially instrumentation for measauring the concentration of carbon monoxide in a gas stream. Of ocurse, measuring techniques and instruments had long existed for this purpose, but frequently they relied on sampling techniques which were too slow to provide useful data for on-line adjustment of combustion parameters, or not reliable enough for continuous duty.
The increased stringency of government regulations relating to power plane emissions has long been a prod for the development of in-situ gas analyzers, and several types of such analyzers have been installed in hundreds of power plants in recent years. Some utilize the technique known as "gas filter correlation", which is a technique utilized in the instant invention. It is an object of this invention to employ this technique to better advantage in a gas analyzer whose sampling is done "in-situ", meaning without removal of a sample from the stream, but instead securing data as the consequence of measurements or observations of spectral energy which has been subjected to interaction with the gas stream itself - either by having passed through the gas stream or by having emanated from it.
Gas filter correlation is a well-known procedure which does not require description here for an understanding of the invention. A useful reference on this subject is "Analytical Methods Applied to Air Pollution Measurements" by Stevens and Herget, Chapter 10, pages 193-231, published by Ann Arbor Science, 1974, which is incorporated by reference herein for its showing of the applicable theory.
This technology, and the instrumentation provided by this invention, are not limited to applications which are sensitive to stack gases, or even only to actively flowing streams of gases. While such applications represent a very large market, there is a growing need to be aware of conditions in what may suitably be called a "bulk" presence of gases. Detection of pollutants and toxic gases in atmosphere is another example, and an extension of this additional application is surveillance and warning of the presence of undesirabale compounds or concentrations of them.
Enclosure and barrier surveillance represents a substantial potential application for this invention. For example, it is useful to know whether a dump or depository is emitting any specific gas or pollutant. In turn, it may be desired only to know the total emission in all directions, in which event a perimeter would be monitored, or in some specific direction in which a barrier would be monitored. In these situations, there is a "stream" of gases being monitored, although not precisely in the sense of a stack gas in which there is a rapid steady flow. Even so, the concepts of this invention are useful to both, and the term "stream" of gases defines both of them.
Gas filter correlation techniques generally utilize narrow band pass filters. In many applications of this invention, it is quite convenient to use for filters, cells containing specific gases at known and precise concentrations and pressures. These techniques are most suitable for detection and analysis of gases whose spectral absorption pattern includes a number of lines in the band of interest, and in which the "interleaved" regions are also utilized in the procedures. Such gases include carbon monoxide and hydrochloric acid.
More classical techniques are used when instead of many absorption lines within the band of interest, there is merely a wide absorption line or band. Then optical notch filters will be employed instead. Examples of such gas are hydrocarbons and carbon dioxide.
The apparatus of this system can utilize either optical filters or gas cells, and the generic terms "filter means" and "filters" is used for both of them. In addition, the sensitivity of the instrument can be improved by providing a narrow band pass filter that limits the energy reaching the detector to those wavelengths that are of interest.
Also, while the measurement of concentration of a selected gas may be of primary interest in many installations, in others the detection of the presence of that gas may be of primary concern, therefore this invention is not intended to be limited to use with measurement devices, but also extends to surveillance and detection devices where the presence or absence of the compound is of interest.
When the term "gas" is used herein, relating to the substance being detected or measured, it is not intended to be limited to compounds in their gaseous state. The measurement or detection of opacity is also comprehended, and this may involve the detection and measurements of particulates conveyed in a gas stream. Such a situation is also intended to be included in the term "gas".
It is an object of this invention to provide a system which can have a direct zero and span measurement, even with gases flowing or present in the apparatus; which can readily and automatically be calibrated, and all interferences automatically rejected; which can be constructed so as readily to be accessed for routine repair and maintenance, and even disposed at a considerable distance from the situs being sampled or observed; which is sufficiently heat resistant that its readings do not stray during temperature excursions; which rejects spurious signals from its surroundings; and which is forgiving of substantial physical shifts and changes in the physical environment, such as by dimensional expansion and contraction.
Still further objects are to provide better techniques for internal calibration of the instrument, for more efficient optical path, and for decreased sensitivity to external physical distortions such as vibratory and temperature induced dimensional shifts.